As it is known, the discs for disc brake comprise a cap or a hub, adapted to associate the disc to a hub of a vehicle, from which an annular portion extends which is called braking band intended to cooperate with pads of a caliper. In the case of the ventilated-type discs, the braking band is obtained by means of two plates respectively facing and mutually connected through connection members, for example in the form of pillars or tabs. The outer surfaces of the two plates define opposite braking surfaces, while the inner surfaces define, in conjunction with the pillars or the tabs, define ventilation ducts for the disc cooling which are passed through by air according to a centrifugal direction during the rotational motion of the same disc.
It is known that the braking action exerted by the pads against the disc braking surfaces generates heat, consequently a temperature increase of the disc, until making the same disc incandescent in the case of a particularly heavy performance. Due to the high temperature which the disc reaches during the braking action, the disc deforms, and the contact between pads and braking surfaces deteriorates. Furthermore, the friction material of the pads undergoes a kind of vitrification and pollution by the disc material.
It has been further found that the highest temperature is reached at a central annular portion of the braking surfaces, i.e. at a central annular portion of the outer surfaces of the respective plates. During the disc life, such area is readily prone to the formation of cracks.
In order to obviate the drawbacks set forth above, therefore, the need to increase the dispersion efficiency of the heat generated by the braking action, so as to limit the temperatures reached by the disc during and following the braking action on one hand, and the need to increase the mechanical strength of these braking band central portions, on the other hand, are particularly felt in the field.
Solutions are known from WO 2004/102028, and also from WO 2002/064992; U.S. Pat. Nos. 7,066,306; 7,267,210; US 2006 0243546; US 2004 0124047; U.S. Pat. Nos. 6,367,599; 5,542,503; and 4,865,167. While being satisfactory from several point of view, these known solutions do not allow reaching a compromise between mechanical strength which is desired in the braking band central annular area and the conflicting need to maximize, in the same area, the air flow able to eliminate the high localized temperature increase which is caused by the braking action.